As Melk-Man said, you can definitely run more classes than you can afford on a 600, so I would go over the event schedule and pick the ones that work for you. For Blackhawk, the schedule is here http://www.ccsracing.us/schedules/2010/gpmw/BHF%20May%20SCHEDULE.pdf
and if I were you I would run the GTU for sure, since it's on Saturday, and then run the Middleweight GP so you'll get two races in on Saturday.
Then pick a couple of races on Sunday that look good, like Middleweight Superbike and Middleweight Supersport. There's plenty of time between the races for you and no one can race them with anything bigger than a 600, so your drive to the finish line will always be against the same size motor.
Best of luck to you, and welcome to the Madness!